1. Field of the Invention
The present invention relates to a field effect transistor (hereinafter denoted as an “FET”) in which a prescribed electrode has a field-plate structure.
2. Description of the Background Art
As is well known, a nitride compound semiconductor material such as GaN, InGaN, AlGaN and AlInGaN has higher band gap energy than GaAs material and, hence, it has high breakdown voltage. Therefore, a device using a nitride compound semiconductor material is superior in operation under high voltage.
Recently, application of an electronic device, particularly an FFT using GaN, to a high voltage semiconductor device such as a power supply device, has been considered promising.
Among FETs using GaN, a high electron mobility transistor (HEMT) has been known, which has a hetero junction structure including a buffer layer formed of GaN, an electron channel layer formed of an undoped GaN and an electron supplying layer formed of an undoped AlGaN thinner than the electron channel layer, formed successively on a semiconductor substrate such as a sapphire substrate.
The HEMT as such includes an FET having a source electrode, a drain electrode and a gate electrode formed on the above-described layer having hetero junction. On a device surface, a highly insulating dielectric film is deposited.
If a high voltage is applied to such a structure, high electric field concentrates on an end of the electrode, resulting in a breakdown.
In order to alleviate the electric field concentration, an FET has been proposed in which a field plate is formed on the dielectric film to alleviate electric field concentration.
FETs as such are disclosed, for example, in US2003-006437 (hereinafter referred to as '437 reference), JP2001-230263 (hereinafter referred to as '263 reference) and US2006-102929 (hereinafter referred to as '929 reference).
The FETs disclosed in '437 and '263 references have a combined structure of a dielectric film and a step-shaped gate field plate. The dielectric film is adapted to have a two-layered structure to attain both high breakdown voltage and high gain.
The FET disclosed in '929 reference has a combination of two types of dielectric films and a gate field plate structure. This structure is to realize higher breakdown voltage and to prevent collapsing.